This invention relates to an interferometer system for measuring the mutual position and movement of a first and a second object in at least one direction, said system comprising, for at least one of all possible mutual directions of movement:
a first interferometer unit, associated with the first object, provided with a fist beam splitter, a first measuring reflector and a plurality of first reflectors, and PA1 a second interferometer unit, associated with the second object, provided with a second beam splitter, a second measuring reflector and a plurality of second reflectors.
The invention also relate to a lithographic apparatus for projecting a mask pattern on a substrate in accordance with the step-and-scan principle.
Such an apparatus, which is known as a step-and-scan apparatus and can be used, inter alia, in the manufacture of integrated circuits, or ICs, is known from, inter alia, U.S. Pat. No. 5,194,893.
Due to the demand for increasing numbers of electronic components in an IC, increasingly smaller details, also referred to as line widths, must be imaged by means of the projection apparatus in every area of the substrate on which an IC must be formed, which area is also referred to as IC area or "die". Moreover, it is also desirable to enlarge the IC areas so that the number of components per IC can also be increased in that way. For the projection lens system, this means that the resolving power, hence its numerical aperture, must be raised and that the image field must be increased.
Hitherto it has been possible, albeit with much trouble and at a high expense, to find an optimum between these two contradictory requirements for a projection lens system. For example, for a stepping apparatus, known as wafer stepper, a projection lens system having a numerical aperture of 0.6 and an image field of 22 mm has been made for the manufacturer of ICs of the 64 Mbit type. Line widths of 0.35 .mu.m can be imaged on the substrate by means of this projection lens system. The limit of a projection lens system which can still be made and is not too unwieldy is then virtually achieved. If even smaller details are to be imaged, i.e., if even smaller line widths are to be formed on the substrates, in other words, if the projection lens system must be given an even larger numerical aperture, then this will only be at the expense of the image field size.
A way out of this dilemma is possible by changing from a stepping projection apparatus to a step-and-scan apparatus as described in U.S. Pat. No. 5,194,893. In a stepping projection apparatus, the full mask pattern is exposed and imaged in one run on an IC area on the substrate. Subsequently, a step is made, i.e. the substrate is moved with respect to the projection lens system and the mask pattern until a second IC area is situated opposite the mask pattern and within the image field of the projection lens system, and a second image of the mask pattern is formed in the area. Subsequently a step is made to a third IC area and imaged again, and so forth until images of the mask patterns have been formed in all IC areas. In a step-and-scan apparatus, the same stepping movements are performed, but every time only a small part of the mask pattern is imaged on corresponding partial area of the substrate. By imaging consecutive parts of the mask pattern on consecutive partial areas of the IC area, an image of the entire mask pattern is obtained on an IC area. To this end, the mask pattern is exposed with a projection beam which has a small cross-section, for example a rectangular or arcuate cross-section, at the area of the mask pattern, and the mask table and the substrate table are moved in opposite sense in a direction, the scanning direction, with respect to the projection lens system and the projection beam, the speed of movement of the substrate table being M times that of the mask table. M is the magnification with which the mask pattern is imaged. A conventional value for M is currently 1/4, but other values, for example 1, are alternatively possible.
The projection beam cross-section has its largest dimension in the direction transverse to the scanning direction. This dimension may be equal to the width of the mask pattern, so that this pattern is imaged in one scanning movement. However, it is alternatively possible that said dimension is half that of the mask pattern or is even smaller. In that case, the entire mask pattern is imaged in two or more opposite scanning movements. It should then be ensured that the movements of the mask and the substrate are synchronized very accurately, i.e. the speed v of the mask should always be equal to M times the speed of the substrate.
As compared with stepping projection apparatus, in which the mask pattern is already aligned accurately with respect to the IC areas on the substrate and in which the projection lens system must be accurately focused on the substrate and the stepping substrate table must be accurately inspected, the condition of speed must be additionally measured in a step-and-scan projection apparatus, in other words, whether the substrate and the mask pattern stand still, as it were, with respect to each other during scan-imaging of the substrate and the mask pattern. Based on this measurement, the speed of one of the tables can then be adapted to that of the other.
In the projection apparatus as disclosed in U.S. Pat. No. 5,194,893, two interferometer systems are used to check the condition of speed. The measuring reflector of the first interferometer system is secured to the substrate table so that the displacement of the substrate table in the scanning direction, hereinafter also referred to as the X direction can be measured with this system. The measuring reflector of the second interferometer system is secured to the mask table, so that the displacement of this table to the scanning direction can be measured with this system. The output signals from the two interferometer systems are applied to an electronic processing unit, for example a microcomputer, in which the signals are subtracted from each other and processed to control signals for the actuators, or driving devices, for the tables.
Due to the high speeds of the tables, required because of the desired large feed-through rate of the substrates through the apparatus, the interferometer signals have a high frequency, or bitrate. When comparing these high-frequency signals, the speed of the processing electronics becomes a limiting factor. Then the delay time, i.e., the time elapsing between the instant when a measurement is performed an the instant when the measuring result becomes available, will play a large role. In a closed servoloop comprising the measuring systems and the actuators or driving devices for the tables, delay time differences in the electronic signal processing will lead to an unwanted offset between the mask table and the substrate table. Moreover, the tables will then have a limited maximum speed.